This invention relates to a valve seat arrangement for a reciprocating machine and more particularly to an improved method of forming a bonded valve seat for an internal combustion engine.
In internal combustion engines, it frequently is the practice to employ aluminum or aluminum alloys as the material for a number of the major engine castings such as the cylinder heads. When the cylinder heads are formed from aluminum or aluminum alloys, however, certain components of the cylinder head are formed from a dissimilar material so as to improve performance. For example, the valve seats of the cylinder head are normally formed from a harder, less heat conductive material such as iron or ferrous iron alloys. By utilizing such harder materials, the valve seat life can be extended. However, the attachment of the dissimilar valve seat insert into the cylinder head presents a number of problems.
Conventionally, it has been the practice to form the cylinder head passages with recesses adjacent the seating area into which the insert rings which form the valve seat are press fit. The use of press fitting has a number of disadvantages. These disadvantages may be understood by reference to FIG. 1 which shows a conventional pressed in type of valve seat.
The cylinder head material 21 is formed with a counter-bore 22 at the cylinder head recess side of the flow passage 23. The flow passage 23 may be either an intake passage or an exhaust passage. The insert ring is indicated by the reference numeral 24 and may be formed from any suitable material, such as a Sintered ferrous material. Such materials have the advantage of having high wear capabilities. After the insert 24 has been pressed into place, its surface is machined as at 25 so as to form the actual valve seating surface.
As may be seen, this technique requires relatively large valve seat inserts in order to withstand the pressing pressures. In addition, the press fit must be such that the insert ring will not fall out when the engine is running. As a result, there are quite high stresses exerted both on the cylinder head and on the insert ring. The stresses can result in loads which may eventually cause cracks in the cylinder head.
These types of construction also limit the maximum size and spacing of the valve seats in order to ensure adequate cylinder head material between adjacent valve seats to reduce the likelihood of cracking. In addition, the large seats compromise the configuration of the intake passages, particularly at the critical valve seating area. Finally, these constructions result in somewhat poor heat transfer from the valve to the cylinder head due to the poor thermal conductivity of the valve seat material and the poor contact area between the insert 24 and the cylinder head 21.
In addition, the interface between the insert ring and the cylinder head frequently leaves voids or air gaps which further reduce the heat transfer and thus cause the valves to run at a higher temperature. This higher temperature operation of the valves requires the valves to be made heavier and stronger and thus reduce the performance of the engine and increase its size and costs.
Many of these problems become worse as the engine reaches operating or higher temperatures. Because of the higher coefficient of expansion of the cylinder head material, the press fit force diminishes and the contact area for heat transfer also decreases.
In addition to these problems which deal primarily with the ultimate performance of the resulting cylinder head and associated engine, there are certain additional manufacturing disadvantages with the pressed-in type of insert. These have to do with the cost and the complicated manufacturing process by which the pressed-in inserts are formed.
As has been previously noted, the insert ring is press-fit into a bore or counterbore 22 which is machined on the cylinder head recess side of the flow passage 23. Because of the techniques which are employed, the positioning and formation of this bore must be done very accurately so as to ensure that the resulting valve seat will be appropriately positioned so that it will cooperate with the associated poppet valve. Thus, a machining operation has been required to accurately form the valve seat receiving counterbore 22. In addition and frequently at the same time, the bore in the cylinder head which receives the respective valve guide also is machined.
Furthermore, to accommodate both the pressing of the valve seat insert and of the valve guide, separate heating steps are required for heating the cylinder head and/or cooling the valve seat insert and the valve guide so as to facilitate installation.
The machining operations require a cleaning operation before the subsequent heating so as to ensure that particles will not be present in the cylinder head that can interfere with the subsequent press-fitting. Thus, the prior art methods are not only expensive but also time-consuming. In addition to resulting in a cylinder head that has less than optimum performance.
To overcome the disadvantages in the performance of the cylinder heads having pressed in inserts it has been proposed, therefore, to employ a technique wherein the insert ring is metallurgically bonded but not alloyed to the cylinder head material. This is accomplished by pressing the insert into place and passing an electrical current through the insert which is sufficient to cause the cylinder head material to plastically deform upon insertion of the insert ring. The plastically deformed phase of the cylinder head material forms a metallurgical bond at the interface with the insert ring without any significant resulting alloying of the cylinder head material to that of the insert ring. Such an arrangement is disclosed in our copending application entitled, "Valve Seat Bonded Cylinder Head and Method for Producing Same," application Ser. No., 08/483,246, filed Jun. 7, 1995 and assigned to the assignee hereof. In addition, certain of these techniques are also described in our copending application entitled "VALVE SEAT," application Ser. No. 08/278,026, filed Jul. 20, 1994, in our names and also assigned to the Assignee hereof.
These techniques have a number of advantages over the conventional structures. First, they permit the use of much smaller insert rings since the pressing pressure is reduced and thus the shape of the intake passage, particularly the shape of the cylinder head passages, particularly in the critical area of the valve seats are not compromised. In addition, the bond strength is considerably higher than more conventional methods. Furthermore, this technique, because of the improved way in which the adhesion is formed, permits the use of much smaller insert rings and thus permits the valve seat openings to be positioned closer to each other without the likelihood of causing defects in the cylinder head which may manifest themselves during the engine running and life.
Therefore it is an object of this invention to provide a further improved method for bonding such valve seat inserts into place.
It is a further object of this invention to provide a further improved method for bonding such valve seat inserts into place that minimizes the machining and other steps required to form the valve seat.